1. Field of the Invention
The present invention relates to a wafer processing method of dividing a wafer into a plurality of devices along a plurality of division lines, the devices being formed on a front side of the wafer and partitioned by the division lines.
2. Description of the Related Art
As well known in the art, in a semiconductor device fabrication process, a functional layer composed of an insulating film and a functional film is formed on a front side of a substrate such as a silicon substrate, and a plurality of semiconductor devices such as ICs and LSIs are formed like a matrix from this functional layer, thus obtaining a semiconductor wafer having the plural semiconductor devices. The plural semiconductor devices are partitioned by a plurality of crossing division lines formed on a front side of the semiconductor wafer. The semiconductor wafer is divided along these division lines to obtain the individual semiconductor devices as chips.
Further, in an optical device fabrication process, an optical device wafer is provided by forming an optical device layer composed of an n-type nitride semiconductor layer and a p-type nitride semiconductor layer on a front side of a sapphire substrate or a silicon carbide substrate. The optical device layer is partitioned by a plurality of crossing division lines to define a plurality of regions where a plurality of optical devices such as light emitting diodes and laser diodes are respectively formed. The optical device wafer is cut along the division lines to thereby divide the regions where the optical devices are formed from each other, thus obtaining the individual optical devices as chips.
As a method of dividing a wafer such as a semiconductor wafer and an optical device wafer along the division lines, there has been tried a laser processing method of applying a pulsed laser beam having a transmission wavelength to the wafer along the division lines in the condition where the focal point of the pulsed laser beam is set inside the wafer in a subject area to be divided. More specifically, this wafer dividing method using laser processing includes the steps of applying a pulsed laser beam having a transmission wavelength to the wafer from one side of the wafer along the division lines in the condition where the focal point of the pulsed laser beam is set inside the wafer to thereby continuously form a modified layer inside the wafer along each division line and next applying an external force to the wafer along each division line where the modified layer is formed to be reduced in strength, thereby dividing the wafer into the individual devices (see Japanese Patent No. 3408805, for example).
In the above method of forming a modified layer inside the wafer along each division line and dividing the wafer along each division line where the modified layer is formed, the modified layer is formed inside the wafer in which the width of each division line is small. Accordingly, it is desirable to apply the laser beam to the wafer from the back side thereof where the devices are not formed. Further, in a pickup step of picking up each device after dividing the wafer along each division line, it is desirable to expose the front side of the wafer where the devices are formed. For these reasons, the wafer is divided in the following manner. First, a laser beam having a transmission wavelength to the wafer is applied to the wafer from the back side thereof along each division line to thereby form a modified layer inside the wafer along each division line. Thereafter, a dicing tape is attached to a back side of the wafer, and a peripheral portion of the dicing tape is supported to an annular frame. Thereafter, an external force is applied to the wafer to thereby divide the wafer into the individual devices (see Japanese Patent Laid-open No. 2006-54246, for example).
However, in the case that the dicing tape is attached to the back side of the wafer after forming the modified layer inside the wafer along each division line, there is a possibility that the wafer may be broken. To cope with this problem, there has been proposed a method such that a front side of a dicing tape is attached to the back side of a wafer and the peripheral portion of the dicing tape is supported to an annular frame prior to forming a modified layer inside the wafer along each division line. Thereafter, a laser beam having a transmission wavelength to the wafer is applied to the wafer through the dicing tape from the back side thereof along each division line in the condition where the focal point of the laser beam is set inside the wafer, thereby forming the modified layer inside the wafer along each division line (see Japanese Patent Laid-open No. 2012-84618, for example).
However, a back side of the dicing tape has fine asperities, causing a problem such that the transmission of the laser beam is hindered and the laser beam cannot be sufficiently focused inside the wafer, so that a desired modified layer cannot be formed. To solve this problem, the technique described in Japanese Patent Laid-open No. 2012-84618 includes a flattening step of applying a liquid resin to the back side of the dicing tape to thereby flatten the back side of the dicing tape. After performing the flattening step, the laser beam having a transmission wavelength to the wafer is applied to the wafer through the dicing tape from the back side thereof along each division line in the condition where the focal point of the laser beam is set inside the wafer, thereby forming the modified layer.